[clang] [clang] Strict aliasing warning ala GCC [PR50066] (PR #74155)

[Nathan Sidwell via cfe-commits]

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@@ -498,3 +498,137 @@ CodeGenTBAA::mergeTBAAInfoForMemoryTransfer(TBAAAccessInfo DestInfo,
   // access type regardless of their base types.
   return TBAAAccessInfo::getMayAliasInfo();
 }
+
+// Determine the aliasing kind bit-converting from type Src to type Dst.
+CodeGenTBAA::AliasingKind CodeGenTBAA::getAliasingKind(QualType &Dst,
+                                                       QualType &Src) {
+  assert(!Src->isVoidType() && !Dst->isVoidType());
+  if (TypeHasMayAlias(Src) || TypeHasMayAlias(Dst))
+    return AK_Ok;
+
+  Src = QualType{Src->getBaseElementTypeUnsafe(), 0};
+  Dst = QualType{Dst->getBaseElementTypeUnsafe(), 0};
+
+  auto *SrcDecl = Src->getAsRecordDecl();
+  auto *DstDecl = Dst->getAsRecordDecl();
+
+  const llvm::MDNode *AnyTBAA = getChar();
+  const llvm::MDNode *SrcTBAA = nullptr;
+  const llvm::MDNode *DstTBAA = nullptr;
+
+  if (!SrcDecl) {
+    SrcTBAA = getTypeInfo(Src);
+    if (!SrcTBAA || SrcTBAA == AnyTBAA)
----------------
urnathan wrote:

Pedantically 'signed char' is not blessed with that property by the std.  But many compilers (llvm included) bestow it upon signed char.  Indeed this particular patch doesn't make a judgement on that, it just gets the aliasing info for 'signed char' and it happens to get the same ubiquitous char tbaa.

https://github.com/llvm/llvm-project/pull/74155